Thanks to everyone above - I had no idea that a micro filter also contained a 1.8uF ringing capacitor. But I had to find out the hard way that the DVA has only a 2 wire cct presentation. I used an old line cord connected to a spare master socket to prove that there was no ringing on the "bell" wire. I thought that it was a configuration error till I saw these posts.
My 30 year old "Viscount", a much younger C&W phone, and a 4 year old Panasonic DECT base station all fell foul of this defective design, and were fixed by an otherwise unnecessary micro filter. Oddly the phone socket on my Smarthub2 is fine for all 3? Left hands and right hands? And of course 0800800150 and the specialist DV team were unable to help at all.